Optical pickup units are mounted on optical disc apparatuses (including magneto-optical disc apparatuses) such as compact discs (or CD), mini discs (or MD) and digital versatile discs (or DVD) apparatuses, etc. so as to record data on optical discs or reproduce the recorded data therefrom.
FIG. 13 shows a schematic diagram illustrating a structure of a conventional optical pickup unit (90), in which an optical pickup lens (10) comprising a lens (or an objective lens) (20) and a lens holder (30), a reflecting mirror (60), a collimator lens (70), and an optical unit (80) are arranged in this order from the side close to an optical disc (50). The optical unit (80) comprises a semiconductor laser as a light source (not shown) for emitting a laser beam, and photo detectors (not shown) which receive reflected light from the optical disc. A laser beam from the semiconductor laser of the optical unit (80) passes through the collimator lens (70), the reflecting mirror (60) and the lens (20) so as to irradiate the optical disc (50). The laser beam reflects on the optical disc (50), and then the reflected laser beam goes backward to be detected by the photo detectors of the optical unit (80).
The lens (20) is mounted on the lens holder (30) which is connected to and is driven by an optical pickup lens-driving mechanism (an actuator, not shown). The optical pickup lens (10) is composed of the lens (20) and the lens holder (30). This arrangement makes it possible to change position of the lens (20) relative to the optical disc (50) by operating the actuator. In other words, it is possible to drive the lens (20) independently in a direction parallel to an optical axis (40) of the lens and in a radial direction of the optical disc (50) (i.e., a direction perpendicular to an optical axis (40) of the lens). Therefore, focusing and tracking can be controlled.
Generally, a plastic lens formed by injection molding is used for the optical pickup lens. The plastic lens is formed by injecting molten plastic into a cavity of a mold through a gate of the mold, cooling and solidifying the same. Therefore, optical distortion (astigmatism) tends to grow in the plastic lens because of an influence of the resin flow generated on the lens being shaped in the mold. Moreover, in case where the mold has a plurality of cavities, subtle difference in the flow of the resin into the cavities of the mold varies the astigmatism of a plurality of the resultant lenses which are shaped in the respective cavities. Furthermore, even if a plurality of plastic lenses are shaped in the same cavity, the astigmatism of the resultant lenses may vary depending on variation in molding conditions for each of lots.
The presence of astigmatism of the optical pickup lens gives an influence on a focus error signal from the optical pickup unit, which may lead to unstable focus servo control. Therefore, careful attention is needed for mounting the lens on the lens holder so that the astigmatism of the lens and the astigmatism of the optical systems except for the lens can be canceled by each other as much as possible, to thereby minimize astigmatism of the whole of the optical pickup unit. In case of mass-production of optical pickup lenses each comprising a lens and a lens holder, an angle for mounting the lens on the lens holder (i.e., an angle for rotating the lens based on an optical axis of the lens as the center of rotation) at which influence of the astigmatism of the lens can be minimized is examined relative to each lot of molded lenses and to each of the cavities of molds, and then, the lens is mounted on the lens holder at the predetermined angle.
Conventionally, adjustment of the angle for mounting the lens (20) on the lens holder (30) is carried out as illustrated in FIGS. 14 to 16. FIG. 14 is a plan view of the optical pickup lens (10), schematically illustrating a method of mounting and positioning the lens (20) on the lens holder (30). FIG. 15 is a sectional view of the optical pickup lens shown in FIG. 14, and FIG. 16 is an exploded perspective view of the optical pickup lens shown in FIG. 14. Positioning of the lens (20) relative to the lens holder (30) in a direction parallel to the optical axis (40) of the lens (20) is carried out by placing a flat portion (22) of a round edge (21) of the lens (20) on a portion (32) of the lens holder (30) which receives the flat portion (22) of the round edge (21) of the lens (20). Further, the positioning of the lens (20) relative to the lens holder (30) in a direction perpendicular to the optical axis (40) of the lens (20) is performed by contacting an outer peripheral side portion (23) of the round edge (21) of the lens (20) with a portion (33) of the lens holder (30) which receives the outer peripheral side portion (23). Accordingly, the positioning of the lens (20) relative to the lens holder (30) can be easily achieved in the directions parallel and perpendicular to the optical axis (40) of the lens (20), respectively.
However, the conventional optical pickup lens (10) has a problem in that any means has not been taken to determine a position for rotating the lens (20) in the direction of rotation (45) of the lens relative to (based on or considering) the center of rotation of the lens (20) (i.e., the optical axis (40)). Therefore, the operation of mounting the lens (20) on the lens holder (30) at the predetermined angle is not easy (cf., JP-A-2001-126275). Further, the conventional optical pickup lens (10) has a problem in that it takes long in mounting the lens (20) on the lens holder (30) at the predetermined angle (cf., JP-A-2001-126275).